Electrostatic discharge (ESD) is harmful to electronic circuits. Generally, an integrated circuit is provided with an electrostatic protection device at an input/output port to discharge the current generated by static electricity, so as to prevent the circuit from being damaged by the static electricity. Since the structure of silicon-controlled rectifier (SCR) possesses ‘snapback’ characteristic, it is generally used as an ESD protection device. SCR has high current-discharging capability and excellent cut-off characteristics; however, the threshold voltage of SCR structure is high. Numerous researches have been made on how to reduce the threshold voltage of the SCR structure. At present, a trigger circuit is usually adopted to reduce the threshold voltage of the SCR structure, and the trigger circuit is generally implemented by means of series-connected diodes or MOSFETs, such as DTSCR, LVSCR, etc. Since the structures of such devices are complicated and the trigger voltages thereof are generally high, it is difficult to meet the requirement of a low supply voltage when the size of the CMOS device is further reduced. Moreover, the area occupied by the protection circuit is relatively large due to the trigger circuit introduced. In order to follow the trend that the size of the CMOS device continues being reduced, it is necessary that the ESD protection devices possess characteristics of a low trigger voltage, a simple structure and a small area, etc.